Geographic Range

Redback spiders are native to all areas of Australia. Redbacks are also found in New Zealand (North and South islands), having been introduced through grape importation from Australia. Records of the spiders’ appearance span most of Southeast Asia, including as far north as India. This species has also recently been sighted in south central Japan. (Garb, et al., 2004; Nihei, et al., 2003; Vink, et al., 2008; Vink, et al., 2010)

Habitat

Redback spiders are most commonly found in urban areas, preferring the shelter human habitats provide from unfavorable weather. They inhabit urban and suburban areas within all of Australia's terrestrial biomes, preferring tropical and temperate areas. They are less common in savanna, chaparral and desert areas and are not found at the continent's highest elevations. The appearance of redbacks in Japan shows that they are also capable of surviving at very low temperatures (-3 degrees C). (Isbister and Gray, 2003; Nihei, et al., 2003; Vink, et al., 2010)

Physical Description

Redback spiders are bilaterally symmetrical, cold-blooded spiders belonging to the family Theridiidae. Their closest relative is the North American southern black widow spider (Latrodectus mactans), which is distinguishable from redback spiders by the absence of a red dorsal stripe. Female redbacks average 10 mm in length, with body sizes as large as a pea, and are significantly larger than males (which average 3-4 mm). Females are typically black with a red stripe, sometimes broken, on the dorsal surface of the upper abdomen (crossing parallel to the length of the body), and a red hourglass-shaped spot on the ventral side of the abdomen. Young female redbacks have additional white markings on their abdomens that they lose as adults. Male redbacks are typically light brown in color with a dorsal red stripe and a paler hourglass shape on the ventral side of the abdomen, both of which are similar to, but less distinct than, female markings. Males also retain the white markings on the upper side of the abdomen through adulthood. Each sex has slender legs and is venomous. (Australian Museum, 2011; Garb, et al., 2004; Vink, et al., 2010)

Development

Young redback spiders follow a similar developmental sequence to other spider species. A week and a half after females lay their egg sacs, the first molt occurs inside individual eggs. The first instar (stage between molts) follows, during which spiderlings hatch and disperse within 14 days, usually via wind currents. Young spiders look like small adults. Members of this species reach maturity and adult size after 4 instars/5 molts (males) or 6 instars/7 molts (females). (Australian Museum, 2011; "Redback spider", 2012; Downes, 1987; Hickman, et al., 2007)

Reproduction

Redback spiders can mate anytime during the year but do so most commonly during summer months when temperatures are higher. Redback spiders are polyandrous (males typically mate once while females often have multiple mates). This species' mating behavior includes sexual cannibalism. Smaller males attempt to mate multiple times with a single, much larger, female. When they approach a female, a male will insert one pedipalp into one of the female's sperm-storage organs. During this action it will twist its body 180 degrees, placing its abdomen directly near the female's fangs (a "copulatory somersault"). Early in this first sperm transfer, the female begins to masticate her partner; most males (69.6-83.3%), are able to break free of this first restraint. After additional courtship behaviors by males, the mating behavior described is repeated, with the male's second pedipalp inserted into the female's other sperm-storage organ. Following 65% of these second insertions, the female will completely consume the male. (Andrade and Banta, 2002; Downes, 1987; Stoltz, et al., 2008)

Some male redbacks have adapted a unique behavior called "mid-dorsal abdominal constriction" in order to increase survival after the first attempt of female cannibalism. This involves manually shrinking their abdomens and, in so doing, shifting essential organs anteriorly, lengthening survival time of males so they may inseminate the female’s second sperm-storage organ. Those who succeed at this behavior increase chances of paternity. (Andrade, et al., 2005)

Due to cannibalistic behaviors by females, a majority of male redbacks only mate with one female. During the mating period, several males are typically found on a female's web, leading males to compete with one another, often fatally, for access to females. Redbacks have a lengthy courtship period of around 3 hours; however, males may rush these activities if another male is detected approaching. If they attempt to shorten courtship too much, females typically cannibalize males before copulation is completed. During copulation, the apical sclerite of the male redback spider's copulatory organ may break off and act as a plug in the female's sperm-storage organs, blocking insemination by other males and helping increase chances of paternity for the first mate. Cannibalized males who exhibit this behavior potentially more than double their likelihood of paternity compared to males not consumed. After consumption of a male, females are much less receptive to further mates. Because 80% of males never find a mate, it is important to invest everything into their one mating experience. Redback spider males who survive copulation are likely sterile for the rest of their lives. (Andrade and Banta, 2002; Snow, et al., 2006; Stoltz, et al., 2008)

Redback spiders in Australia can breed at any time of year, but breeding is most common during summer months. After females have mated, they may use internally stored sperm for up to 2 years to fertilize their eggs. During this time they lay multiple batches of eggs, from different supplies of sperm, with a period of at least 1-3 weeks between each batch. Batches are made up of about 10 egg sacs, each of which contains approximately 250 eggs, which are laid and suspended in the web. When sacs are laid they are white but after time turn brown. The length of time before hatching is related to temperature. It has been recorded that they emerge after 17-24 days at 30°C and after 26-43 days at 25°C. The average duration of egg to spiderling emergence is 28.7 days. Below 25°C, development is typically arrested. Once emerged, sexual organs start soon begin developing, with female redback spiders reaching sexual maturity after 120 days and the males after 90 days. (Australian Museum, 2011; Downes, 1987; Vink, et al., 2010)

Information on parental investment for this species is limited. Females lay large numbers of eggs and suspend them in bundles from their webs, possibly as a form of protection. When hatched, spiderlings quickly disperse from their mother's territory. Because male redbacks typically do not survive past mating, it is assumed that no male parental investment occurs. (Andrade and Banta, 2002; Australian Museum, 2011)

Lifespan/Longevity

Female Redback spiders live for 2-3 years whereas males only live for about 6-7 months. Male lifespan is limited by sexual cannibalism during mating, male-male competition, and size differential between males and females (often leading to females killing the much smaller males). (Australian Museum, 2011; Downes, 1987; Stoltz, et al., 2008)

The presence of Redback spider populations in Japan shows that they are capable of surviving without food for long periods of time, as they likely traveled the long distance from Australia to Japan in cargo carried by ships with little to no food available. Juveniles may survive up to 160 days and adults 300 days without food. At later stages of starvation they are sluggish and incapable of finding food for themselves, however Redbacks are noted to recover immediately after one meal. (Forster and Kavale, 1989; Nihei, et al., 2003)

Typical lifespanStatus: wild

6 to 36 months

Behavior

This is a shy, nocturnal species, that rarely interacts with conspecifics, except when mating. Redback spiders stay out of sight during the day, preferring to hide in dry interior environments, such as sheds, where there is shelter and plenty of prey. If outside, they live under rocks, logs, or low growing plants. Like most spiders, female redbacks spin their own unique webs made with a strong silk (males are not able to make webs). Their webs are a disorganized funnel shape. The spiders are stationary most of the time, perched in the very rear of the funnel. The web is constructed in such a way that they are able to feel the vibrations made by prey running into their traps. (Australian Museum, 2011; BBC Science & Nature, 2008; Main, 2001; Vink, et al., 2010)

Redback spiders in Japan hibernate during the cold winter months. This is a behavior not witnessed in any other part of the world that these spiders inhabit. (Nihei, et al., 2003)

Home Range

Redback spiders are largely sedentary animals, preferring to stay within a very small home range. Sometime before adulthood, spiderlings disperse from their place of birth by catching air currents while attached to long silk threads, with their landing places typically determining their nesting areas. (Australian Museum, 2011; Vink, et al., 2010)

Communication and Perception

Webs are constructed so that spiders are able to sense vibrations, using their hair-like setae, made by animals running into the web's strands. Unmated female redbacks deposit a chemical pheromone on their webs to attract males and, during courtship, male redbacks must make their presence known using tactile cues to avoid being consumed. As with most spiders, redbacks have simple eyes that are capable of sensing movement. (Australian Museum, 2011; BBC Science & Nature, 2008; Hickman, et al., 2007; Jerhot, et al., 2010)

Food Habits

Redback spiders are insectivores that prey on small insects caught within their webs such as ants (Formicidae spp). They also sometimes catch larger animals stuck in their webs such as mice, small birds, snakes, small lizards, king crickets, Cromwell chafer beetles, and trapdoor spiders. Redback spiders also steal stored prey caught in the webs of other spiders. All redbacks have a potentially venomous bite, however only females have been known to envenomate prey. (Australian Museum, 2011; BBC Science & Nature, 2008; Queensland Museum, 2007)

Redback spiders catch their prey in a unique way. At night, females construct a complex web system reaching in all directions, including towards the ground. They set traps for prey by bringing a strand of their silk web down and sticking it to the ground surface. Next, they climb up that line, adding an additional silk line on top of the original to strengthen it. They then pull the line taut and a single trap is complete. They do this multiple times creating a number of traps and wait for prey to run or fly into a line and get stuck. Redbacks eventually wrap up each prey item, storing some for later meals. (BBC Science & Nature, 2008)

Economic Importance for Humans: Positive

While redback spiders are predators of terrestrial insects, they do not appear to affect insect populations enough to have any positive effects on humans. (Isbister and Gray, 2003)

Economic Importance for Humans: Negative

Redback spiders are one of the most dangerous spiders in Australia. When disturbed, larger females often bite and envenomate intruders. Bites typically occur during the summer season and in the late afternoon when temperatures are highest and the spiders are most active. The venom is produced in the cephalothorax and is delivered via the animal's fangs. Redback spiders can control the amount of venom that they inject and "dry" bites are not unheard of. The main toxic component of the venom, α-latrotoxin, affects humans differently depending on the amount injected. Males are believed to be as capable of delivering painful, venomous bites as females, although bites are rarely reported. Approximately 80% of bites cause little to no effect. Of the remaining 20%, most feel pain radiating from the bite spot for only about 24 hours. More serious cases include pain lasting longer than 24 hours, bumps and swollen lymph nodes followed by sweating, a rapid heart beat, possible vomiting, headache, and insomnia. Unlike most other envenomation syndromes, the effects of bites from this species may persist for several days, weeks, or months. Fatalities from redback bites are rare and none have occurred since 1956, when an antivenom was created. Most bites from this species can be treated with household remedies (ice packs and pain relievers). Bites that show more severe symptoms may require antivenom to be administered intramuscularly, sometimes in multiple injections. Recently, Australian doctors have chosen to only give the antivenom when absolutely necessary because they fear possible negative side effects. Other doctors are not convinced of its effectiveness in general. (Isbister and Gray, 2003; Isbister, 2002; Nimorakiotakis and Winkel, 2004; Wiener, 2003)

Contributors

Glossary

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

aposematic

having coloration that serves a protective function for the animal, usually used to refer to animals with colors that warn predators of their toxicity. For example: animals with bright red or yellow coloration are often toxic or distasteful.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chaparral

Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.

chemical

uses smells or other chemicals to communicate

delayed fertilization

a substantial delay (longer than the minimum time required for sperm to travel to the egg) takes place between copulation and fertilization, used to describe female sperm storage.

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

female parental care

parental care is carried out by females

fertilization

union of egg and spermatozoan

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

insectivore

An animal that eats mainly insects or spiders.

internal fertilization

fertilization takes place within the female's body

introduced

referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

polyandrous

Referring to a mating system in which a female mates with several males during one breeding season (compare polygynous).

rainforest

rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

sperm-storing

mature spermatozoa are stored by females following copulation. Male sperm storage also occurs, as sperm are retained in the male epididymes (in mammals) for a period that can, in some cases, extend over several weeks or more, but here we use the term to refer only to sperm storage by females.

stores or caches food

places a food item in a special place to be eaten later. Also called "hoarding"

suburban

living in residential areas on the outskirts of large cities or towns.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

urban

living in cities and large towns, landscapes dominated by human structures and activity.

venomous

an animal which has an organ capable of injecting a poisonous substance into a wound (for example, scorpions, jellyfish, and rattlesnakes).

vibrations

movements of a hard surface that are produced by animals as signals to others

Disclaimer:
The Animal Diversity Web is an educational resource written largely by and for college students. ADW doesn't cover all species in the world, nor does it include all the latest scientific information about organisms we describe. Though we edit our accounts for accuracy, we cannot guarantee all information in those accounts. While ADW staff and contributors provide references to books and websites that we believe are reputable, we cannot necessarily endorse the contents of references beyond our control.

This material is based upon work supported by the
National Science Foundation
Grants DRL 0089283, DRL 0628151, DUE 0633095, DRL 0918590, and DUE 1122742. Additional support has come from the Marisla Foundation, UM College of Literature, Science, and the Arts, Museum of Zoology, and Information and Technology Services.